1. Field of the Invention
This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2007-120377, filed on Apr. 28, 2007, the disclosure of which is incorporated herein in its entirety by reference.
The present invention relates to a radio communications system having a plurality of radio zones (hereinafter, referred to as cells) and, more particularly, to a method and device for resource allocation control in the system.
2. Description of the Related Art
In a mobile communications system, for a base station and a mobile station to perform data communication, they need to establish synchronization between them in advance. Since initial access from the mobile station in particular is not always in synchronization, the base station requires some procedure for uplink synchronization with the mobile station.
In LTE (Long Term Evolution), which is being standardized by the 3rd generation partnership project (3GPP), a random access channel (PACH) and an uplink shared channel (UL-SCH) are provided for uplink synchronization and uplink data transmission. The RACH is a channel to transmit a control signal for establishment of uplink synchronization, and further to request a resource for transmission of uplink data. To establish uplink synchronization without a long delay, it is preferable that RACH transmission collision probability be reduced as low as possible (see 3GPP TS 36.300 V.1.0.0, Mar. 19, 2007). On the other hand, the UL-SCH is a channel to transmit data and Layer-2/Layer-3 control packets. Since the transmission power of the UL-SCH in particular increases in accordance with the transmission rate, it is necessary to take account of interference with other signals.
FIG. 1A is a diagram schematically showing a generic structure of a mobile communications system according to the LTE, and FIG. 1B is a resource structure diagram schematically showing radio resources based on both frequency division and time division. Here, a mobile station is labeled “UE”, which is an abbreviation of User Equipment. It is assumed that a mobile station UE1 located in a cell A of a base station eNB1 transmits data to the base station eNB1 through UL-SCH, and that a mobile station UE2 located in a cell B of a base station eNB2 transmits a control signal to the base station eNB2 through RACH.
In wideband code division multiple access (WCDMA), RACH and EDCH (Enhanced dedicated Channel) share the same frequency resource, multiplexed by using different spreading and scrambling codes. On the other hand, in the LTE, a plurality of frequency-divided and time-divided resources are shared by RACH and UL-SCH exclusively of each other. Specifically, the LTE uplink has a resource structure in which a system bandwidth of 10 MHz is time-divided into time intervals of 1 msec, each of which is further frequency-divided into widths of 1.25 MHz. Referring to FIG. 1B, each of t1, t2, . . . on the horizontal axis corresponds to a 1-msec-long time resource, and each of FB#1, FB#2, . . . on the vertical axis corresponds to a 1.25-MHz-wide frequency resource. Hereinafter, one rectangular block defined by one time resource and one frequency resource as shown in FIG. 1B will be simply referred to as “resource.”
It is each base station eNB that determines how to allocate such system resources to the RACH and UL-SCH. In general, a RACH resource is periodically allocated as shown in FIG. 1B so that a mobile station UE can gain access to the base station eNB without a long delay. It is also possible to allocate a plurality of RACH resources at a time and thereby secure a sufficient RACH access capacity. Each base station eNB generally broadcasts information indicative of which resource(s) is allocated to the RACH. Therefore, in accordance with the broadcast information, every mobile station UE in a cell can gain access to the RACH resource(s) whenever the mobile station UE needs. Within a cell, the RACH and UL-SCH do not directly interfere with each other as long as RACH resources and UL-SCH resources are allocated in accordance with the frequency division and time division shown in FIG. 1B.
However, in the case where different control entities individually perform resource allocation in neighboring cells, there is a possibility that a PACH transmission in one of the cells interferes with an UL-SCH transmission in the other cell. As mentioned above, since each base station eNB, on its own responsibility, individually allocates uplink resources of the cell under its control to the RACH and UL-SCH, there is a case where a high-speed uplink data transmission in a certain cell interferes with a RACH transmission in a neighboring cell. For example, when a call setup procedure is started, the prevention of this RACH interference is particularly important, considering that uplink synchronization needs to be established through RACH transmission. This is because, if the data transmission in the certain cell interferes with the RACH transmission in the neighboring cell, call setup will be delayed in the neighboring cell. This is not limited to the case of call setup. In the case where RACH transmission is required prior to the start of other processing as well, a similar effect is caused: specifically, the processing is delayed due to strong uplink interference of a high-speed uplink data transmission to the RACH transmission.
FIGS. 2A and 2B are schematic diagrams showing an example of inter-cell interference. Here, it is assumed that frequency resources are allocated to the RACH and UL-SCH in each of cells A and B independently, as shown in FIG. 2A. If these cells A and B are as far away from each other as they do not affect each other, interference is not problematic. However, if the cells A and B are neighboring cells as shown in FIG. 1A and the frequency band and timing of data transmission performed by a mobile station UE1 located in the cell A coincide with those of RACH transmission performed by a mobile station UE2 located in the cell B, then the possibility is large that interference as shown in FIG. 2B occurs.
For example, referring to FIG. 2B, when the mobile station UE1 in the cell A desires to transmit data at a certain point of time, the mobile station UE1 first performs RACH transmission using a RACH resource designated by a broadcast signal from the base station eNB1. Then using an UL-SCH resource granted by means of a response from the base station eNB1, the mobile station UE1 starts uplink data transmission. At this time, assuming that the mobile station UE2 similarly starts RACH transmission to transmit data by using a resource that coincides with the UL-SCH used in the cell A, the base station eNB2 cannot detect the RACH transmission from the mobile station UE2, due to interference with the UL-SCH transmission of the mobile station UE1 in the cell A. If no response is received from the base station eNB2, the mobile station UE2 increases transmission power and repeats RACH transmission. After a response (GRANT) is received from the base station eNB2, the mobile station UE2 starts UL-SCH transmission. As described above, the possibility increases that the start of communication of the mobile station UE2 is greatly delayed due to a collision with the UL-SCH transmission.
To avoid such interference, a conceivable method is that a resource that makes the smallest interference is searched for and this smallest-interference resource is allocated to RACH transmission, as described in Japanese Patent Application Unexamined Publication No. 2002-526970, for example. However, according to this method, a RACH resource cannot be allocated periodically, leading to unstable access from a mobile station UE to a base station eNB, resulting in a high possibility of a long delay. Moreover, since a resource making small interference is generally allocated to data transmission, the fact that the smallest-interference resource is allocated to RACH transmission may cause larger interference in reverse, making it easier for collision to occur.
Such a problem concerns not only the LTE, but may exist in general radio communications systems using an access scheme (FTDMA) based on a frequency-divided and time-divided resource structure.